The present invention relates to a voice coil-type linear motor used as a means for positioning a magnetic head in a magnetic disk drive, etc.
A voice coil-type linear motor has a simple structure and is excellent as a power source of high-speed linear motion, so it is used in a large varieties of applications. Particularly, in a magnetic disk drive, it is necessary that a magnetic head moves from one track position to another on a magnetic disk at high speed and with high accuracy. Therefore, as a means for positioning a magnetic head, a voice coil-type linear motor having shorter access time than other motors is widely used.
Generally, a voice coil-type linear motor is constituted by a cylindrical outer yoke, a center yoke, a permanent magnet fixed to an inner surface of the outer yoke and a movable coil(see, for instance, U.S. Pat. Nos. 3,723,779, 3,723,780 and 4,305,105).
The permanent magnets for such a motor are usually Alnico magnets, ferrite magnets, rare earth cobalt magnets, etc. In a voice coil-type linear motor, the thickness of a permanent magnet is restricted such that its operating point cannot be made high(generally, its permeance coefficient is 0.8-3 or so, which may vary depending upon the magnetic circuit structure). Accordingly, it has been necessary to use ferrite magnets or rare earth cobalt magnets having high coercive force. Particularly, since voice coil-type linear motors are required to be smaller and have higher performance with the miniaturization and increase in performance of magnetic disk drives, rare earth cobalt magnets have been widely used(see for instance, Japanese Patent Laid-Open No. 56,74077).
Recently, because of the demand for higher magnetic flux density in the magnetic gap in a voice coil-type linear motor, rare earth-iron-boron (R-Fe-B) permanent magnets have become widely used in such a voice coil-type motor or a linear motor (European Patent Publication 0 101 552, Japanese Patent Laid-Open Nos. 61-210862 and 61-266056).
However, when an R-Fe-B rare earth permanent magnet is incorporated into a voice coil-type linear motor, conventionally a large number of arc magnet segments are used, and these magnet segments are bonded to an inner wall of an outer cylindrical yoke to form a cylindrical magnet. However, this entails a high cost. And when a plurality of arc magnet segments are bonded to form an integral cylindrical magnet, the magnetic flux density in the magnetic gap of a magnetic circuit fluctuates by about 8-10% because adjacent magnet segments are bonded to each other with some circumferentially extensive radial bonding areas for ensuring a sufficient bonding strength. Therefore, the movement of the movable coil has limited accuracy.
In addition, since an R-Fe-B rare earth permanent magnet contains large amounts of rare earth elements(particularly, Nd) and iron which are easily oxidizable in the air, the magnet is likely to be covered by an oxide layer on the surface when used in a magnetic circuit, leading to a decrease in the magnetic flux density in a gap. Therefore, in a case for using an R-Fe-B permanent magnet, it is usual to form an anti-oxidizing layer on the surface by various methods(see, for instance, Japanese Patent Laid-Open Nos. 60-153109, 61-130453, 61-150201, etc.). However, since a magnetic gap is as narrow as few millimeters in a voice coil-type linear motor, the magnet is likely to be chipped by contact with a coil when it is assembled in the motor, even though the magnet is covered with an anti-oxidizing layer on the surface. Accordingly, assembling effiency was conventionally sacrificed to prevent the chipping of the magnet.